-The RSPAN Vlan needs to exist
in the Vlan database of the source switch, the destination switch and all
switches in the transit path between them. It also needs to be allowed on all
Trunk ports between the source and destination switches.

1. Select Device > Setup > Operations and Export "Export named configuration snapshot."
2. Select the XML file that contains your running configuration (for example, running-config.xml) and click OK to export the configuration file.
3. Save the exported file to a location external to the firewall. You can use this backup to restore the configuration if you have problems with the upgrade.

STEP 2 : Make sure the firewall is running content release needed for the installation of required OS

1.Select Device > Dynamic Updates.
2.If the firewall is not running the minimum required update, Check Now to retrieve a list of available updates.
4.Locate and Download the appropriate update.
5.After the download completes, Install the update.

STEP 3: Determine the upgrade path.

1.Select Device > Software > Check now for the latest update
2.Locate and Download the version to which you intend to upgrade.
3.After the download completes, Install the update.
4.After the installation successfully completes, reboot using one of the following methods:
If you are prompted to reboot, click Yes.
If you are not prompted to reboot, select Device > Setup > Operations and Reboot Device (Device Operations section).

NOTE : You cannot skip installation of any major releases in the path to your target PAN-OS version. Therefore, if you intend to upgrade to a version that is more than one major release away, you must still download, install, and reboot the firewall for each intermediate major release along the upgrade path.

For example, if you want to upgrade from PAN-OS 6.0.11 to PAN-OS 7.1.5, you must:

Download and install PAN-OS 6.1.0 and reboot.
Download and install PAN-OS 7.0.1 and reboot (7.0.1 is the base image for the 7.0 release, not 7.0.0).
Download PAN-OS 7.1.0 (you do not need to install it).
Download and install PAN-OS 7.1.5 and reboot.

Launch the Firepower Chassis Manager Web Interface from browser using https://<chassis_mgmt_ip_address> [This is the IP address of the Firepower 9300 that you entered during initial configuration] and login

If you have more than one chassis configure it in the same way using different management IP address.